Solid carbonaceous subterranean formations such as coal seams can contain significant quantities of natural gas. This natural gas is composed primarily of methane. The majority of the methane contained within a solid carbonaceous subterranean formation is adsorbed to the carbonaceous material of the formation. In order to recover the methane from the formation, the pressure within the formation's cleats must be reduced. This will cause methane to desorb from the methane sorption sites and diffuse to the cleats. Once within the cleat system, the methane can flow to a recovery well where it is recovered.
In addition to methane, solid carbonaceous subterranean formations often contain large quantities of water. Typically, to provide a satisfactory methane recovery rate from a recovery wellbore, the region of the formation surrounding the recovery wellbore must be dewatered to lower the pressure within the cleats to a point where sufficient quantities of methane are desorbing from the methane adsorption sites. This dewatering is achieved by reducing a recovery wellbore's pressure to establish a differential pressure between the reservoir pressure of the formation and the wellbore. The differential pressure established will cause the water to flow from the cleats to the recovery wellbore. As the water is removed from the cleat system and the pressure in the cleats is reduced, the methane recovery rate will increase. The recovery of methane from a solid carbonaceous subterranean formation which is controlled by the lowering of the pressure within the cleat system caused by the removal of methane and other fluids from a recovery well is generally referred to as "primary pressure depletion methane recovery."
Once within the wellbore, the water should be removed so that a backpressure is not applied to the formation. A backpressure on the formation can reduce the methane recovery rate from the wellbore or, in some instances, may completely inhibit the flow of methane from the wellbore. This can be a problem especially where the formation is underpressured or undersaturated.
Methane also may be recovered from solid carbonaceous subterranean formations using techniques which take advantage of the reduction in the partial pressure of methane which occurs within the cleats when a gaseous desorbing fluid is injected into the formation. Techniques which enhance the recovery of methane from a solid carbonaceous subterranean formation by the use of an injected gaseous desorbing fluid are hereinafter referred to as "enhanced methane recovery techniques;" such techniques are generally described in U.S. Pat. No. 5,014,785 to Puri et al. As with primary pressure depletion methane recovery, the methane recovery rate may be reduced if water is not removed from a recovery wellbore during enhanced methane recovery.
In general, the water production rate tends to decrease over time as the solid carbonaceous subterranean formation dewaters. However, later in a wellbore's serviceable life, the need to remove water from the wellbore may be greater because the reservoir pressure of the formation typically will be lower, and therefore water in the wellbore can more easily inhibit the flow of methane from the formation.
Small pieces of carbonaceous material, hereinafter referred to as "coalfines," often slough into a wellbore over time. These coalfines can plug up the wellbore and impede the recovery of methane from the wellbore. The coalfines can also plug up equipment which may be used to dewater the wellbore. Therefore, coalfines also must often be removed from a wellbore during the wellbore's serviceable life.
Various lift techniques have been used to remove the water from a wellbore. One technique uses a conventional gas-lift design to lift water out of the wellbore. In a typical gas-lift design, a chamber is formed within the wellbore by setting a wellbore packer above the carbonaceous seam which is located closest to the surface. The chamber's volume is defined by the volume within the wellbore below the packer. A first tubing string set into the packer carries pressurized lift-gas to the chamber. A second tubing string, which passes through the packer to a location near the bottom of the wellbore, transports the water from the wellbore to the surface when pressurized lift-gas is injected into the chamber. One deficiency of such a design is that the pressurization of the chamber by lift-gas places a backpressure on the face of the formation which impedes the flow of methane into the wellbore and thereby reduces the methane recovery rate from the wellbore. Furthermore, many solid carbonaceous subterranean formations comprise several carbonaceous seams which are vertically interspersed with layers of sandstone and other noncarbonaceous materials. These carbonaceous seams may be vertically distributed along the wellbore with the deepest carbonaceous seam located near the bottom of the wellbore and the shallowest carbonaceous seam located near the surface of the earth. In these types of formations, a lift-gas pressure which would be sufficient to remove water from the bottom of the wellbore may completely prevent methane located within the upper carbonaceous seams from flowing into the wellbore.
Another type of gas-lift design uses a prefabricated chamber which is lowered into the wellbore to collect water that flows into the wellbore. An example of such a design is contained in U.S. Pat. No. 5,211,242 to Coleman et al. The apparatus disclosed in Coleman et al. is designed to intermittently remove water from the wellbore to the surface. It comprises a chamber for collecting water from the formation, two individual tubing strings, and a standing valve which is used to isolate the chamber from the surrounding wellbore. One of the tubing strings is used to carry pressurized lift-gas to the chamber; the other tubing string is used to transport water from the chamber to the surface. The standing valve is designed to close when pressurized lift-gas is injected into the chamber, thereby isolating the chamber from the surrounding wellbore region. While this apparatus is effective for removing water intermittently from the wellbore, it does not provide a method for continuously removing water from the formation. It also does not readily allow for removal of coalfines from the bottom of the wellbore, requiring instead the use of a workover rig to first remove the apparatus and associated tubing, before the coalfines can be removed from the wellbore.
What is desired is an apparatus and method which is capable of removing water either continuously or intermittently from a wellbore. Preferably, the apparatus should be capable of being easily switched between the continuous and intermittent modes of operation without the need for a workover rig. Further, the apparatus should be constructed so that coalfines can be easily and efficiently removed from the chamber and the bottom of the wellbore, without the need of a workover rig.
As used herein, the following terms shall have the following meanings:
(a) "carbonaceous material" refers to the solid carbonaceous materials that are believed to be produced by the thermal and biogenic degradation of organic matter. The term carbonaceous material specifically excludes carbonates and other minerals which are believed to be produced by other types of processes; PA1 (b) "cleats" or "cleat system" is the natural system of fractures within a solid carbonaceous subterranean formation; PA1 (c) a "coalbed" comprises one or more coal seams in fluid communication with each other through a wellbore; PA1 (d) "coal seams" are carbonaceous formations which typically contain between 50 and 100 percent organic material by weight; PA1 (e) "coiled tubing" refers to a continuous length of tubing which can be stored on a reel. The coiled tubing is unreeled from the reel and run into a wellbore. Coiled tubing and its associated handling equipment typically consist of a tubing roll, storage injector heads to move the coiled tubing into or out of the wellbore, power unit and control assembly, and pressure control equipment; PA1 (f) "gaseous desorbing fluid" includes any fluid or mixture of fluids which is capable of causing methane to desorb from a solid carbonaceous subterranean formation; PA1 (g) "kick-off pressure" refers to the hydrostatic head exerted on the bottom of a wellbore as a result of the water present within the wellbore just prior to a gas-lift apparatus of the current invention being installed within the wellbore; PA1 (h) "longitudinal cross-sectional area" refers to the area defined by the inner volume of a tube or body which lies within a plane that is perpendicular to the longitudinal axis of the wellbore. For a tube which has a longitudinal axis which is parallel to the wellbore's longitudinal axis, the inner volume is defined by the inside diameter of the tube; PA1 (i) "reservoir pressure" means the pressure at the face of the productive formation when the well is shut-in. The reservoir pressure can vary throughout the formation. Also, the reservoir pressure may change over time as fluids are produced from the formation and/or gaseous desorbing fluid is injected into the formation; PA1 (j) "seating nipple" refers to a member configured to accept a valve body. The seating nipple mates with the body of the valve. Typically, a seating nipple comprises a short piece of tubing which has an internal diameter which is slightly smaller than the internal diameter of a tubing to which it is typically coupled; PA1 (k) "solid carbonaceous subterranean formation" refers to any substantially solid carbonaceous, methane-containing material located below the surface of the earth. It is believed that these methane-containing materials are produced by the thermal and biogenic degradation of organic matter. Solid carbonaceous subterranean formations include but are not limited to coalbeds and other carbonaceous formations such as antrium, carbonaceous, and devonian shales; PA1 (l) "wireline" is a strong length of wire that typically is between 0.070 and 0.092 inches in diameter and is typically mounted on a powered reel at the surface of the earth near a wellbore. The wireline is used to transfer wireline retrievable tools into the wellbore. The wireline is often guided into the wellbore with a mast which aids in the alignment of the wireline within the center of the wellbore; PA1 (m) "wireline retrievable tools" are tools which can be transferred into and out of a wellbore using a wireline. Wireline retrievable tools can be used to perform such tasks as wellbore depth measurement, fishing for lost parts and junk retrieval, and the manipulation and installation of downhole fluid flow control devices; and PA1 (n) "workover rig" refers to a rig which is used to insert and remove tubular piping sections from a wellbore. A workover rig includes a derrick and associated pipe handling gear. Workover rigs are typically used for inserting and pulling a sectional tubing string from a wellbore and for installing artificial water lift equipment. PA1 a) installing a gas-lift apparatus in the wellbore, the gas-lift apparatus comprising: PA1 b) operating the gas-lift apparatus in a continuous water removal mode; PA1 c) switching the gas-lift apparatus from the continuous water removal mode to an intermittent water removal mode using a wireline retrievable tool; and PA1 d) operating the gas-lift apparatus in the intermittent water removal mode. PA1 a chamber for collecting water, the chamber having an upper and a lower end; PA1 a valve receiving means for receiving a valve coupled to the lower end of the chamber; PA1 a water transport tubing for transporting water from the chamber, the water transport tubing having a lower end coupled to the upper end of the chamber; PA1 a linear axis means for transferring a wireline into and at least partially through the chamber which is formed by an axial alignment of the lower end of the water transport tubing and the chamber about the longitudinal axis of the wellbore; and PA1 coiled tubing located externally to the water transport tubing, the coiled tubing having an upper end coupled to the wellhead and having a lower end operatively coupled to the chamber for conducting pressurized lift-gas from the wellhead to the chamber to facilitate the removal of water from the chamber. PA1 casing set within the wellbore for conducting the methane to the surface of the earth; PA1 perforations which penetrate the casing in regions of the wellbore which are adjacent to the at least one coal seam, the perforations allowing methane to travel from the at least one coal seam into the wellbore; PA1 a chamber located within a portion of the casing, the chamber having an upper end and a lower end and having an maximum longitudinal cross-sectional area; PA1 a seating nipple coupled to the lower end of the chamber, the seating nipple being configured for receiving a valve and having a longitudinal cross-sectional area which is smaller than the chamber's maximum longitudinal cross-sectional area; PA1 a water transport tubing located within the casing for transporting water to the surface of the earth, the water transport tubing having a lower end coupled to the upper end of the chamber, the lower end of the water transport tubing having a longitudinal cross-sectional area which is smaller than the chamber's maximum longitudinal cross-sectional area, the lower end of the water transport tubing, the chamber, and the seating nipple being axially aligned about the longitudinal axis of the wellbore to form a linear access means for transferring a wireline into and at least partially through the chamber; and PA1 coiled tubing operatively coupled to the chamber and located within the casing and externally to the water transport tubing for conducting pressurized lift-gas from the earth's surface to the chamber to facilitate the removal of water through the water transport tubing to the earth's surface.